This invention relates to a low supply voltage oscillator circuit, and, more specifically, this invention relates to a circuit that can generate an oscillating signal that is independent of the supply voltage, temperature of operation and the process used to create the circuit, and can work with a low supply voltage.
Oscillator circuits are widely used in analog and digital circuits. They can be employed in a large number of applications, such as for driving a voltage multiplier, or generating a clock frequency or a programmable delay.
A good oscillator is usually expected to be operable at a high switching frequency and to have an oscillation frequency independent of the supply voltage, process variations, and temperature. It is also expected to have a duty cycle which can be defined for a constant ratio, and to exhibit low electromagnetic interference (EMI), that is, primarily voltages edges with controlled slopes of moderate steepness.
In recent years, there has been an increased demand for devices capable of operating on lower supply voltages, e.g., for cellular telephone and computer applications, and increased demand for more efficient performance, e.g., for the processing of signals in a synchronous state machine, which must be managed at a very high clock frequency.
A very simple oscillator circuit 1 is shown in FIG. 1. It is formed by connecting into a loop two inverters INV1xe2x80x2, INV2xe2x80x2, a resistor R0, and a capacitor C0, and generates an oscillating electric signal of substantially square shape.
The oscillator circuit 1 has the following frequency of oscillation:                               f          0                =                  1                                    R              0                        ⁢                                          C                0                            ·                              [                                                      ln                    ⁡                                          (                                                                                                    V                            S                                                    +                                                      V                            TH                                                                                                    V                          TH                                                                    )                                                        +                                      ln                    ⁡                                          (                                                                                                    V                            S                                                    +                                                      V                            TH                                                                                                                                V                            S                                                    -                                                      V                            TH                                                                                              )                                                                      ]                                                                        (        1        )            
where,
VS is the supply voltage to the oscillator circuit 1; and
VTH is the switching voltage threshold of the inverters INV1xe2x80x2 and INV2xe2x80x2.
This simple circuit fails, however, to meet the aforementioned requirements. In particular:
the oscillation frequency is dependent on the switching voltage threshold of the inverters: its value reaches a maximum when the switching voltage threshold of the inverters is one half the supply voltage;
the duty cycle of the oscillator circuit 1 also is dependent on the switching voltage threshold of the inverters: its value is 50% when the switching voltage threshold of the inverters is one half the supply voltage; and
the voltage at the input of the first inverter INV1xe2x80x2 has larger variations than the circuit supply voltage: its value varies between VTH+VS and VTHxe2x88x92VS.
From an article by S. Hobrecht, xe2x80x9cAn Intelligent BiCMOS/DMOS Quad 1-A High-Side Switchxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 25, No. 6, Dec. 1990, especially from FIG. 4 and its description on page 1397, a more sophisticated oscillator circuit 2 is known, as shown schematically in FIG. 2.
The oscillator circuit 2 includes a first inverter INV1 and a second inverter INV2 which are cascade connected to each other and connected to an output terminal OUT of the oscillator circuit 2, and two similar symmetrical sections connected to form a double feedback loop between the input terminal of the first inverter INV1 and the output terminal OUT of the oscillator circuit 2.
A first of the sections includes a first transistor SW1 of the P-channel MOS type and a first current generator GEN1, which are connected in series with each other between a supply terminal VDD and a ground voltage reference GND. In addition, the first transistor SW1 has a control terminal connected to the output terminal OUT.
The first section also includes a first capacitor C1 connected in parallel with the first current generator GEN1 and connected to the ground reference GND, and includes a first controlled switch SW3 which is connected between the first capacitor C1 and the input terminal of the first inverter INV1.
To provide for the circuit feedback, the control terminal of the first controlled switch SW3 is connected to an interconnection node X between the first INV1 and second INV2 inverters.
Likewise, the oscillator circuit 2 includes a second section, which in turn includes a second capacitor C2 having first and second terminals, a second current generator GEN2 having an input terminal and an output terminal, a second transistor SW2 of the N-channel MOS type, and a second controlled switch SW4, in a circuit configuration which is similar to that just described for the first section.
Where the two capacitors C1 and C2 have the same capacitance Co and the generators GEN1 and GEN2 have the same current value Io, the oscillation frequency of the oscillator circuit 2 will be:                     fo        =                  Io                                    V              S                        ·            Co                                              (        2        )            
Thus, the oscillator circuit 2 is not affected by the problems brought about by dependence of the oscillation frequency on the switching voltage threshold of the inverters, and on a varying input voltage to the first inverter.
However, not even this solution is entirely devoid of drawbacks. In particular:
the oscillation frequency is dependent on the supply voltage VDD;
the duty cycle is dependent on the switching voltage threshold of the inverters; and
the oscillation frequency and duty cycle are both affected by any asymmetry existing between the current generators GEN1 and GEN2, since they are formed in practice by P-channel and N-channel transistors.
A further approach is described in European Patent No. 0 735 677 to this Applicant, herein incorporated by reference, wherein, as shown schematically in FIG. 3, a single capacitor C is used which is charged and discharged by two current generators (Gen1, Gen2 ) having first and second values, such that the voltage across the capacitor C will follow a triangular pattern and have an amplitude which corresponds substantially to the ratio of the product of the two values and their sum.
In particular, the oscillator circuit 3 includes a capacitor C, charge circuitry CCA, and control circuitry CCO.
The charge circuitry CCA includes the first and second current generators, GEN1 and GEN2, which generate two current values with opposite signs to deliver current at the output of the circuitry CCA and absorb it. It also includes a switch means, represented by first SW1 and second SW2 switches operative to alternately couple the generators GEN1, GEN2 to the capacitor C.
The control circuitry CCO has a voltage input coupled to the capacitor C and an output coupled to control inputs of the switches SW1, SW2, and includes a comparator with hysteresis.
An oscillating signal appears at several points of the oscillator circuit 3 which can be utilized to output, for example, the voltage across the capacitor C. However, it is convenient to have the circuit output OUT connected to the output of the comparator with hysteresis, that is, to the output of the circuitry CCO, so that a substantially square wave-like form can be obtained. This square wave normally needs no buffering because the output of a comparator has relatively low impedance.
The circuitry CCO has an input and an output, and includes two comparators COMP1, COMP2, an inverter COMP3, two controlled switches SW3, SW4, and first and second voltage generators VTH and VTL.
The oscillator circuit 3 has essentially two operational conditions: a first condition which corresponds to the capacitor C being injected the current from the generator GEN1, and a second condition which corresponds to the capacitor C being extracted the current drawn by the generator GEN2.
The circuitry CCO is adapted to alternatively activate the first or the second operational condition according to whether the voltage across the capacitor C has dropped below the lower threshold or exceeded the upper threshold.
The oscillation frequency of the oscillator circuit 3 is substantially:                               f          0                =                  1                                                    R                0                            ⁢                              C                0                                      ⁢                          xe2x80x83                                                          (        3        )            
Thus, the oscillation frequency and duty cycle of the oscillator circuit 3 are dissociated from the supply voltage, the temperature and the process.
Although in many ways advantageous, this approach cannot be used in low supply voltage applications.
Embodiments of this invention provide an oscillator circuit suitable for low supply voltage applications which has such structural and functional features as to overcome the drawbacks that still beset prior art devices.
Specifically, these embodiments relate to a low supply voltage oscillator circuit that has at least one capacitor to be controlled, connected between first and second voltage references, and a circuit for charging and discharging the controlled capacitor. The oscillator can be formed with CMOS technology, for instance.
One embodiment of the invention includes two capacitors being charged alternately in a controlled fashion with the intermediary of a memory device, such as a bistable device or an SR flip-flop.
The features and advantages of an oscillator circuit according to the invention will be more clearly understood from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings.